IEEE 802.11 Wireless Local Area Networks are considered a wireless version of the Ethernet, with the current 802.11 standard supporting only best-effort services. In anticipation of real-time applications, such as voice over WLANs and video streaming, the IEEE 802.11 work group established the 802.11e task group. The aim of this group is to enhance the current 802.11 IEEE MAC protocol and to enable it to support applications with QoS requirements.
At the conceptual core of 802.11e is a Hybrid Coordination Function (HCF), that combines with and enhances some aspects of contention-based and contention-free channel access method outlined in the 802.11 standard, to provide QoS stations (QSTAs) with prioritized and parameterized access to the wireless medium (WM), while continuing to support non-QoS stations (STAs) for best-effort transfers. The two channel access mechanisms are enhanced distributed channel access (EDCA) and HCF controlled channel access (HCCA) mechanisms. With HCF, the basic unit of allocation for the right to transmit in the WM is the transmission opportunity (TXOP). Each TXOP is defined by a starting time and a defined maximum length. The TXOP is obtained either by the station with quality of service (QSTA) by successfully contending for the channel (EDCA-TXOP), or is assigned by the Hybrid Channel HC (HCCA-TXOP).
The HCF clause disclosed by the draft amendment to the standard for telecommunications and information exchange between systems—LAN/MAN, 802.11e, Specific Requirements section, currently describes the QoS enhancements to the MAC in functional terms. Accordingly, QSTAs may access the channel in a more controlled manner than a non-QSTA, and transmit the message protocol data units (MPDUs). The TXOP may be obtained by a QSTA winning an instance of EDCA contention during the contention period (CP), or by a non-AP QSTA receiving a QoS (+)CF-Poll during the CP or contention free period (CFP). The former is EDCA-TXOP, while the latter is HCCA-TXOP or polled TXOP. The draft amendment referred to prior in the document, prescribes that a TXOP shall not extend across a target beacon transmit time (TBTT) or exceed the dot11MaxDwellTime (if using an FH PHY). The occurrence of a TBTT implies the end of the TXOP, after which the regular channel access procedure (EDCA or HCCA) is resumed. It is possible that no frame is transmitted during the TXOP. The foreshortened termination of the TXOP does not imply an error condition. It translates in lost transmission opportunities and unnecessary power consumption, therefore shortening the battery life of the station.
What are needed are means that allow the participation of the stations in the frame transmission without having to contend for the channel and unnecessarily use power while doing so. Therefore, what are needed are means that allow a cooperative participation of the stations in the frame transmission. What are also needed are means that allow access points to have tighter control of the timing of access to the wireless medium for scheduled service. What are further needed are means that reduce jitter for scheduled services and induce power savings for stations that wake-up for scheduled services, especially for the stations operating in power saving mode.